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UNIT 2 Cardiovascular Measurements 1 DEEPAK.P Objective At the end of this Unit You will learn Different Biomedical measurements such as ECG, Blood pressure measurement, Cardiac Measurements DEEPAK.P 2 Cardiac Function Measurements 3 DEEPAK.P Measuring Cardiac Function 4 1. Blood Pressure 2. Electrocardiogram 3. Stress Test 4. Angiography DEEPAK.P Measuring Cardiac Function 1. Blood Pressure Measure of fluid pressure within system a. Systolic Pressure: Pressure generated by contraction b. Diastolic Pressure: Pressure achieved between contractions. SBP reflects the amount of work the heart is performing DBP indicates the amount of peripheral resistance encountered 5 DEEPAK.P Blood Pressure Measurement 6 DEEPAK.P Blood Pressure Measurements Adequate blood pressure is essential to maintain the blood supply and function of vital organs. A history of blood pressure measurements has saved many person from death by providing warnings of dangerously high blood pressure (hypertension) in time to provide treatment. The maximum pressure reached during cardiac ejection is called Systole. Minimum pressure occurring at the end of ventricular relaxation is called diastole. 7 DEEPAK.P Blood Pressure Measurements In routine clinical tests, blood pressure is usually measured by means of an indirect method using a sphygmomanometer (from the Greek word, sphygmos, meaning pulse). This method is easy to use and can be automated. The automated indirect method of B.P measurement is called Electro sphygmomanometer 8 DEEPAK.P Blood Pressure Measurements It has, however, certain disadvantages in that it does not provide a continuous recording of pressure variations and its practical repetition rate is limited. Blood pressure is measured in millimeters of mercury (mm Hg) and recorded with the systolic number first, followed by the diastolic number. A normal blood pressure would be recorded as 120/80 mm Hg. 9 DEEPAK.P Blood Pressure Measurements 10 DEEPAK.P Blood Pressure Measurements The systolic pressure is the maximum pressure in an artery at the moment when the heart is beating and pumping blood through the body. The diastolic pressure is the lowest pressure in an artery in the moments between beats when the heart is resting. Both the systolic and diastolic pressure measurements are important If either one is raised, it means you have high blood pressure (hypertension). 11 DEEPAK.P Blood Pressure Measurements The nominal values in the basic circulatory system Arterial system-------30-300mmHg Venous system--------5-15mmHg Pulmonary system----6-25mmHg Blood pressure measurement can be classified in to Indirect 2. Direct 1. 12 DEEPAK.P Blood Pressure Measurements Indirect Simple equipment ,Very little discomfort, Less informative and Intermittent The indirect method is also somewhat subjective, and often fails when the blood pressure is very low (as would be the case when a patient is in shock). 13 DEEPAK.P Indirect Blood Pressure Measurement 14 DEEPAK.P Blood pressure measurements 1. Auscultatory Auscultatory method uses aneroid sphygmomanometer with a stethoscope. The auscultatory method comes from the Latin word "listening. 2. Oscillometric The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure which are caused by the oscillations of blood flow, i.e., the pulse. 15 DEEPAK.P Blood pressure measurements 3. Palpatory Physician identifies the flow o blood in the arteries by feeling the pulse 16 DEEPAK.P 1. 17 B.P measurements using sphygmomanometer DEEPAK.P Blood pressure measurements using sphygmomanometer First, a cuff is placed around your arm and inflated with a pump until the circulation is cut off. A small valve slowly deflates the cuff, and the doctor measuring blood pressure uses a stethoscope, placed over your arm, to listen for the sound of blood pulsing through the arteries. That first sound of rushing blood refers to the systolic blood pressure; once the sound fades, the second number indicates the diastolic pressure. 18 DEEPAK.P Blood pressure measurements using sphygmomanometer 19 DEEPAK.P Blood pressure measurements using sphygmomanometer 20 DEEPAK.P Direct Blood Pressure Measurement 21 DEEPAK.P 2. Direct Blood Pressure Measurements Provide continuous measurement Reliable information Transducers are directly inserted in to the blood stream Methods for direct blood pressure measurement, on the other hand, do provide a continuous readout or recording of the blood pressure waveform and are considerably more accurate than the indirect method 22 DEEPAK.P Direct B.P Measurement Methods of direct blood pressure were classified in to two 1. The clinical method by which the measuring device was coupled to the patient 2. Second, by the electrical principle involved. First category is expanded, with the electrical principles involved being used as four subcategories. 23 DEEPAK.P Direct B.P Measurement 24 1. A catheterization method involving the sensing of blood pressure through a liquid column. In this method the transducer is external to the body, and the blood pressure is transmitted through a saline solution column in a catheter to this transducer DEEPAK.P Direct B.P Measurement 25 2. The catheterization method involving the placement of the transducer through a catheter at the actual site of measurement in the blood stream or by mounting the transducer on the tip of the catheter. 3. Percutaneous methods in which the blood pressure is sensed in the vessel just under the skin by the use of a needle or catheter. 4. Implantation techniques in which the transducer is more Permanently placed in the blood vessel or the heart by surgical methods. DEEPAK.P B.P measurements using direct method ln l972, Hales inserted a glass tube into the artery of a horse and crudely measured arterial pressure. Regardless of the electrical or physical principles involved, direct measurement of blood pressure is usually obtained by one of three methods Percutaneous insertion. Catheterization (vessel cut down). lmplantation of a transducer in a vessel or in the heart. 26 DEEPAK.P B.P measurements using direct method ln l972, Hales inserted a glass tube into the artery of a horse and crudely measured arterial pressure. Regardless of the electrical or physical principles involved, direct measurement of blood pressure is usually obtained by one of three methods 27 1. Percutaneous insertion. 2. Catheterization (vessel cut down). 3. Implantation of a transducer in a vessel or in the heart. DEEPAK.P 1. Percutaneous insertion ( direct method) Typically, for Percutaneous insertion , a local anesthetic is injected near the site of invasion. The vessel is occluded and a hollow needle is inserted at a slight angle towards the vessel. When the needle is in place, a catheter is fed through the hollow needle , usually with some sort of a guide. When the catheter is securely place in the vessel, the needle and guide are withdrawn. 28 DEEPAK.P Percutaneous insertion ( direct method) For some measurements, a type of needle attached to an airtight tube is used, so that the needle can be left in the vessel and the blood pressure sensed directly by attaching a transducer to the tube. Other types have the transducer built in-the tip of the catheter. This latter type is used in both percutaneous and catheterization models. 29 DEEPAK.P B.P measurements using direct method ln l972, Hales inserted a glass tube into the artery of a horse and crudely measured arterial pressure. Regardless of the electrical or physical principles involved, direct measurement of blood pressure is usually obtained by one of three methods Percutaneous insertion. Catheterization (vessel cut down). lmplantation of a transducer in a vessel or in the heart. 30 DEEPAK.P 2. Catheterization( direct method) It was first developed in the late 1940s and has become a major technique for analyzing the heart and other components. Catheter is a long tube that is inserted in to the heart or major vessels. Sterilized catheters are used Apart from obtaining blood pressures in the heart chamber and great vessels, this technique is also used to obtain blood samples from the heart for oxygen-content analysis and to detect the location of abnormal blood flow pathways. 31 DEEPAK.P Catheterization( direct method) Measurement of blood pressure with a catheter can be achieved in two ways. In the first method is to introduce a sterile saline solution into the catheter so that the fluid pressure is transmitted to a transducer out side the body. In the second method, pressure measurements are obtained at the source. Here,the transducer is introduced into the catheter and pushed to the point at which the pressure is to be measured. or the transducer is mounted at the tip of the catheter. 32 DEEPAK.P Catheterization( direct method) 33 DEEPAK.P Catheterization( direct method) This device is called a catheter-tip blood pressure transducer. For mounting at the end of a catheter, one manufacturer uses an un bonded resistance strain gage in the transducer, whereas another uses a variable inductance transducer . Implantation techniques involve major surgery. Transducers can be categorized by the type of circuit element used to sense the pressure variations, such as capacitive, inductive, and resistive. Since the resistive types are most frequently used. 34 DEEPAK.P Heart 35 DEEPAK.P Heart The cardiovascular system is made of the heart, blood and blood vessels 36 DEEPAK.P Anatomy of the Heart The human heart is a four-chambered muscular organ The heart is enclosed in a pericardial bag. The purpose of it is to protect and lubricate the heart. The peircardium is the outermost covering of your heart. It protects against friction rubs and protects against shocks(traumatic) as it contains 40-50 ml of pericardial fluid. It acts as a shock absorber 37 DEEPAK.P Anatomy of the Heart Heart normally pumps 5 liters of blood per minute Two side of the wall is separated by the septum or dividing wall of tissue. This septum include AV node Right auricle is lies between inferior(lower) and superior(upper) vena cava At the junction of Superior vena cava and right atrium SA node is situated. 38 DEEPAK.P . Anatomy of the Heart The communication between atria and ventricle is accomplished only through AV node and delay line. The activated AV node, after a delay, initiates an impulse in to the ventricle, through the bundle of his, and bundle branches that connect to the purkinje fibers. Ventricle wall is thicker than auricular wall 2. Left atrium is smaller than Right atrium 3. Left ventricle is considered as most important. 4. It wall thickness is 3 times than right ventricle. 1. 39 DEEPAK.P Heart anatomy Left heart is considered as pressure pump Right heart is similar to a volume pump Muscle contraction of left heart is larger and stronger than that of right heart. 40 DEEPAK.P Heart circulation The work of the heart is to pump blood to the lungs through pulmonary circulation and to the rest of the body through systemic circulation. In pulmonary circulation, the pressure difference between arteries and veins is small. In systemic circulation, the pressure difference between arteries and veins is very high. 41 DEEPAK.P Heart Valves The pumping action is accomplished by systematic contraction 42 and relaxation of the cardiac muscle in the myocardium. Cardiac muscles gets the blood supply from coronary circulation. Heart contains 4 valves Tricuspid---Between RA and RV----- Three cups Pulmonary/Semi lunar-- Between RV and Right lungs Mitral/Bicuspid--- Between LA and LV---- Two cups Aortic--- Between LV and aorta The sounds associated with the heartbeat are due to vibrations in the tissues and blood caused by closure of the valves. DEEPAK.P Heart valves 43 DEEPAK.P Heart Sound Listening of sound produced by heart is called auscultation Heart sound is heard by the physician through his stethoscope. This sound is called Korotkoff sound The sounds associated with the heartbeat are due to 44 vibrations in the tissues and blood caused by closure of the valves. Normal heart produces two sounds called lub-dub Lub is called the first heart sound It occurs at the time of QRS complex of the ECG Lub is related to the closure of atrioventricular valve Which permits blood flow from auricle to ventricles. It prevents blood flow in reverse direction DEEPAK.P Heart Sound Dub is called the second heart sound Dub is related to the closure of semilunar valve This valve releases blood into the pulmonary and systemic 45 circulation system. It occurs at the end of the T wave of of the ECG Abnormal heart sounds is called murmurs. It is due to the improper opening of the valve. Graphic recording of heart sound is also possible It is called phonocardiogram Recording of the vibrations of the heart against thoracic cavity is called vibrocariogram DEEPAK.P Cardiac Output and Rate 46 DEEPAK.P Cardiac Output Cardiac output is the volume of blood pumped by the heart per minute (mL blood/min). Cardiac output is a function of heart rate and stroke volume. Cardiac Output in mL/min = heart rate (beats/min) X stroke volume (mL/beat) Cardiac Output = 70 (beats/min) X 70 (mL/beat) = 4900 mL/minute. The total volume of blood in the circulatory system of an average person is about 5 liters (5000 mL). 47 DEEPAK.P Cardiac Output The heart rate is simply the number of heart beats per minute. This can be easily measured through the use of heart rate monitors or taking ones pulse (counting the ‘pulses’ at the radial artery for example over a one minute period). Children (ages 6 - 15) 70 – 100 beats per minute Adults (age 18 and over) 60 – 100 beats per minute 48 DEEPAK.P Cardiac Output The stroke volume is the volume of blood, in milliliters (mL), pumped out of the heart with each beat. Stroke volume (SV) refers to the quantity of blood pumped out of the left ventricle with every heart beat. If the volume of blood increased (waste products not being removed to the kidneys due to kidney failure for example) then there would be a greater quantity of blood within the system increasing the pressure within. 49 DEEPAK.P Cardiac Output Increasing either heart rate or stroke volume increases cardiac output. 50 DEEPAK.P ECG 51 DEEPAK.P Electro Cardio Gram(ECG) Bio electric potentials generated by heart muscles are called Electro Cardio Gram. It is sometimes called EKG(Electro Kardio Gram) Electrocardiography (ECG) is an interpretation of the electrical activity of the heart over a period of time. The recording produced by this noninvasive procedure is termed as electrocardiogram (also ECG or EKG). 52 DEEPAK.P Introduction to ECG measurement system 53 DEEPAK.P Early ECG measurement system 54 DEEPAK.P Electro Cardio Gram(ECG) Heart is divided in to 4 chamber Upper chamber------ Atria( left and right) Lower chamber------Ventricles(left and right) Right auricles receives blood from the veins and pump in to right ventricles. The right ventricle pump the blood to lungs, where it is oxygenated The oxygenated blood enters in to left auricle. Left auricle pumps blood in to left ventricle. To work the cardiovascular system properly , the atria and ventricles must operate in a proper time relationship. 55 DEEPAK.P Electro Cardio Gram(ECG) Action potential in the heart originates near the top of the right 56 atrium at a point called pacemaker or sinoatrial node (S.A node). This action potential is then propagated in all directions along the surface of both atria. The waves terminate at a point near the centre of the heart is called A.V node(Atrioventricular node) At this point some special fiber act as a delay line to provide proper timing between the action of auricles and ventricles. Once electrical pulses has passed through the delay line , it is spread to all parts of both ventricles by the bundle of His It is called purkinje fibers. DEEPAK.P Electro Cardio Gram(ECG) This bundle is divided in to two branches to initiate action potential simultaneously in the two ventricles. ECG waveform/ PQRST wave form 57 DEEPAK.P Electro Cardio Gram(ECG) The “P” wave is called base line or isopotential line. P wave ----- De polarization of Auricles. Combined QRS wave---- Re-polarization of atria and depolarization of ventricles T wave ----- Ventricular re polarization U wave --- after potentials in the ventricles P-Q interval – Time during which excitation wave is delayed in the fiber near AV node. 58 DEEPAK.P Electro Cardio Gram(ECG) 59 DEEPAK.P ECG 60 DEEPAK.P ECG 61 DEEPAK.P ECG 62 DEEPAK.P ECG Recorder 63 DEEPAK.P ECG 64 DEEPAK.P ECG measurement system 65 DEEPAK.P ECG measurement system The ECG system comprises four stages, each stage is as following: (1)The first stage is a transducer—AgCl electrode, which convert ECG into electrical voltage. The voltage is in the range of 1 mV ~ 5 mV. (2) The second stage is an instrumentation amplifier (Analog Device, AD624), which has a very high CMRR (90dB) and high gain (1000), with power supply +9V and -9V. (3) We use an opto-coupler (NEC PS2506) to isolate the In-Amp and output. (4) After the opto-coupler is a bandpass filter of 0.04 Hz to 150 Hz filter. It’s implemented by cascading a low-pass filter and a high pass filter. 66 DEEPAK.P Simple Block diagram of ECG 67 DEEPAK.P ECG Machine 68 DEEPAK.P ECG Leads 69 DEEPAK.P EKG Leads Leads are electrodes which measure the difference in electrical potential between either: 1. Two different points on the body (bipolar leads) 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads) Placement of ECG electrode 71 DEEPAK.P Eintovan’s triangle 72 DEEPAK.P EKG Leads The standard EKG has 12 leads: 3 Standard Limb Leads 3 Augmented Limb Leads 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart. Eintovan’s triangle 74 DEEPAK.P ECG Leads Two types of Leads 75 DEEPAK.P Standard Limb Leads Standard Limb Leads 77 1. Lead I = (VLA - VRL) - (VRA - VRL) = VLA – VRA 2. Lead II = (VLL - VRL) - (VRA - VRL) = VLL – VRA 3. Lead III = (VLL - VRL) - (VLA - VRL) = VLL - VLA DEEPAK.P Standard Limb Leads Augmented Limb Leads 79 DEEPAK.P Augmented Limb Leads Chest Leads Chest Leads Unipolar (+) chest leads (horizontal plane): Leads V1, V2, V3: (Posterior Anterior) Leads V4, V5, V6:(Right Left, or lateral) The 6 leads are labelled as "V" leads and numbered V1 to V6. They are positioned in specific positions on the rib cage. 82 DEEPAK.P All Limb Leads All Leads Leads Waveform ECG Amplifier 86 DEEPAK.P ECG Amplifier We measure the ECG by connecting two electrodes on the right and left chest respectively, as shown. The body should be connected to ground of the circuits, so that we connect the leg to the ground. To boost the raw ECG signal level without boosting the noise amplifiers are used. An electronic circuit should amplify the potential difference across a lead 87 DEEPAK.P ECG Amplifier 88 DEEPAK.P ECG Amplifier 89 DEEPAK.P Instrumentation Amplifier 90 DEEPAK.P Instrumentation Amplifier 91 DEEPAK.P Practical Instrumentation Amplifier 92 DEEPAK.P Instrumentation Amplifier Low signal noise Very high open-loop gain Very high common-mode rejection ratio Very high input impedance Instrumentation amplifier can reduce common-mode noise, but not completely 93 DEEPAK.P Phonocardiogram 94 DEEPAK.P Heart Sound Hippocrates (460-377 BC) provided the foundation for auscultation when he put his ear against the chest of a patient and described the sounds he could hear from the heart. The biggest breakthrough in auscultation came in 1816 when René Laennec (1781-1826) invented the stethoscope 95 DEEPAK.P Heart Sound There are two types of sounds High frequency sounds associated with closing and opening of the valves and Low frequency sounds related to early and late ventricular filling events. 96 DEEPAK.P Heart Sound Mitral area: 2. Tricuspid area: 3. Aortic area: 4. Pulmonic area: 1. Microphones and accelerometers are the natural choice of sensor when recording sound. 97 DEEPAK.P Heart Sound 1. The first heart sound (S1) – systolic sound: Appears at 0.02 – 0.04s after the QRS complex the “lub” frequency of 30-40Hz 2. The second heart sound (S2) – diastolic sound Appears in the terminal period of the T wave the “dub” frequency of 50-70 Hz 3. 98 The third heart sound (S3) - protodiastolic sound Low frequency DEEPAK.P Heart Sound 4. The fourth heart sound (S4) – presistolic sound Appears at 0.04s after the P wave (late diastolic-just before 99 S1) Low frequency S1 – onset of the ventricular contraction S2 – closure of the semilunar valves S3 – ventricular gallop S4 – atrial gallop DEEPAK.P Phonocardiography Graphic recording of heart sound is called phonocardiogram ( PCG) Phonocardiography, diagnostic technique that creates a graphic record, or phonocardiogram, of the sounds and murmurs produced by the contracting heart, The phonocardiogram is obtained either with a chest microphone or with a miniature sensor in the tip of a small tubular instrument that is introduced via the blood vessels into one of the heart chambers. 100 DEEPAK.P Phonocardiography 101 DEEPAK.P Phonocardiography 102 DEEPAK.P Defibrillator 103 DEEPAK.P Defibrillator Defibrillation is a process in which an electronic device gives an electric shock to the heart. This helps reestablish normal contraction rhythms in a heart having dangerous arrhythmia or in cardiac arrest. In recent years small portable defibrillators have become available. These are called automated external defibrillators or AEDs. Defibrillation is a common treatment for life-threatening ventricular fibrillation tachycardia. 104 DEEPAK.P and pulse less ventricular Defibrillator Defibrillators were first demonstrated in 1899 by Jean-Louis Prévost and Frédéric Batelli, two physiologists from University of Geneva, Switzerland. These early defibrillators used the alternating current from a power socket, transformed from the 110–240 volts available in the line, up to between 300 and 1000 volts, to the exposed heart by way of "paddle" type electrodes. 105 DEEPAK.P Defibrillator Early successful experiments of successful defibrillation by the discharge of a capacitor performed on animals were reported by N. L. Gurvich and G. S. Yunyev in 1939. 106 DEEPAK.P Defibrillator Principles 107 DEEPAK.P Defibrillator Principles 108 DEEPAK.P Defibrillator Principles There are many types of defibrillators Monophasic, 2. Biphasic and 3. Internal. The first two types are known as external defibrillators, and these are used on the exterior of the patient’s chest. 1. Pads are placed on the chest and a button is pushed to send an electrical current to the heart. The type of external defibrillator determines the type of current sent to the heart. 109 DEEPAK.P Defibrillator Principles 110 DEEPAK.P Defibrillator Principles A monophasic defibrillator sends out a single electrical pulse. This shot of electricity goes from one pad to the other with the heart in between. A monophasic defibrillator needs high electricity levels to function correctly. The charge is typically started at 200 joules and increased to 300 joules; if necessary, the highest level is 360 joules. 111 DEEPAK.P Defibrillator Principles 112 DEEPAK.P Defibrillator Principles The second type of external device is biphasic, and it sends out two electrical currents. A current first travels from one pad to the other. The electricity then reverses direction and returns a current to the first pad. This enables the biphasic device to use less electricity than the monophasic variety. The biphasic defibrillator also is able to adjust to the patient's body type. 113 DEEPAK.P Defibrillator Principles The third type of defibrillator is the internal or implantable variety, which is surgically placed in the chest of a patient. The electrode wires are inserted through the veins into the right chamber of the heart. An internal defibrillator monitors the heartbeat for any irregularities. Internal defibrillators run on battery power instead of electricity. 114 DEEPAK.P Block Diagram of Defibrillator 115 DEEPAK.P Pacemaker 116 DEEPAK.P Pacemaker A pacemaker (or artificial pacemaker, so as not to be confused with the heart's natural pacemaker) is a medical device that uses electrical impulses, delivered by electrodes contracting the heart muscles. The primary purpose of a pacemaker is to maintain an adequate heart rate, Modern pacemakers are externally programmable and allow the cardiologist to select the optimum pacing modes for individual patients. 117 DEEPAK.P Pacemaker 118 DEEPAK.P Pacemaker Doctors recommend pacemakers for many reasons. The most common reasons are bradycardia and heart block. Bradycardia is a heartbeat that is slower than normal. Heart block is a disorder that occurs if an electrical signal is slowed or disrupted as it moves through the heart. Heart block can happen as a result of aging, damage to the heart from a heart attack, or other conditions that disrupt the heart's electrical activity. 119 DEEPAK.P Pacemaker A pacemaker consists of a battery, a computerized generator, and wires with sensors at their tips. (The sensors are called electrodes.) The battery powers the generator, and both are surrounded by a thin metal box. The wires connect the generator to the heart. A pacemaker helps monitor and control your heartbeat. 120 DEEPAK.P Pacemaker The electrodes detect your heart's electrical activity and send data through the wires to the computer in the generator. The two main types of programming for pacemakers are demand pacing and rate-responsive pacing. A demand pacemaker monitors your heart rhythm. It only sends electrical pulses to your heart if your heart is beating too slow or if it misses a beat. 121 DEEPAK.P Pacemaker A rate-responsive pacemaker will speed up or slow down your heart rate depending on how active you are. To do this, the device monitors your sinus node rate, breathing, blood temperature, and other factors to determine your activity level. People may need a pacemaker for a variety of reasons — mostly due to one of a group of conditions called arrhythmias, in which the heart's rhythm is abnormal. 122 DEEPAK.P Pacemaker During an arrhythmia, the heart may not be able to pump 123 enough blood to the body. This can cause symptoms such as fatigue (tiredness), shortness of breath, or fainting. Severe arrhythmias can damage the body's vital organs and may even cause loss of consciousness or death. A pacemaker can often be implanted in your chest with a minor surgery. You may need to take some precautions in your daily life after your pacemaker is installed. DEEPAK.P Pacemaker Types There are three types of artificial pacemakers Single chamber pacemakers set the pace of only one of your Heart s chamber s , usually the left ventricle , and need just one lead Dual chamber pacemakers set the pace of two of your hearts chambers and need two leads Dual chamber pacemakers are ideal if you have heart block Biventricular pacemakers use three leads, one in the right atrium (one of the top pumping chambers in your heart) and one in each of the ventricles (left and right) 124 DEEPAK.P Ballistocardiograph 125 DEEPAK.P Ballistocardiograph Ballistocardiography (BCG) is based upon Newton's Third Law, which states that for every action there is an equal and opposite reaction. Ballistocardiography, graphic recording of the stroke volume of the heart for the purpose of calculating cardiac output. BCG measures cardiac output by means of recoil forces. With each systole, blood is ejected through the aorta. There are two basic types of ballistocardiographic methods. In the older method, high-frequency BCG, the subject is restrained and force is measured by displacement of a supporting spring. 126 DEEPAK.P Ballistocardiograph In ultra-low-frequency BCG, the subject is free to move and force is calculated from his/her mass and the measured acceleration. Typically, in obtaining a BCG the subject lies on a light, frictionless table which is either suspended from the ceiling or supported from below on an air cushion. The movements of this ballistotable, resulting from body movements produced by cardiac activity, are transduced into electrical energy by means of either mechanoelectronic tubes (Geddes & Baker, 1968) or a compound transducer in which movement of the table is converted into a varying light intensity 127 DEEPAK.P Ballistocardiograph Methods 128 DEEPAK.P Ballistocardiograph Dock and Taubman (1949) recorded body movements without the use of a ballistotable by devising a photoelectric transducer which was attached to the shins of the subject. Cardiac-induced body movements alter the transmission of light to these photoelectric detectors, thus producing a variable electrical output proportional to movement. 129 DEEPAK.P